While a number of efforts are currently being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology.
One technology that has attracted a good deal of attention recently utilizes as the deep UV light source a high-intensity KrF excimer laser, especially an ArF excimer laser featuring a shorter wavelength. There is a desire to have a microfabrication technique of finer definition by combining exposure light of shorter wavelength with a resist material having a higher resolution.
In this regard, the recently developed, acid-catalyzed, chemical amplification type resist materials are expected to comply with the deep UV lithography because of their many advantages including high sensitivity, resolution and dry etching resistance. The chemical amplification type resist materials include positive working materials that leave the unexposed areas with the exposed areas removed and negative working materials that leave the exposed areas with the unexposed areas removed.
In chemical amplification type, positive working, resist compositions to be developed with alkaline developers, an alkali-soluble phenol or a resin and/or compound in which carboxylic acid is partially or entirely protected with acid-labile protective groups (acid labile groups) is catalytically decomposed by an acid which is generated upon exposure, to thereby generate the phenol or carboxylic acid in the exposed area which is removed by an alkaline developer. Also, in similar negative working resist compositions, an alkali-soluble phenol or a resin and/or compound having carboxylic acid and a compound (acid crosslinking agent) capable of bonding or crosslinking the resin or compound under the action of an acid are crosslinked with an acid which is generated upon exposure whereby the exposed area is converted to be insoluble in an alkaline developer and the unexposed area is removed by the alkaline developer.
On use of the chemical amplification type, positive working, resist compositions, a resist film is formed by dissolving a resin having acid labile groups as a binder and a compound capable of generating an acid upon exposure to radiation (to be referred to as photoacid generator) in a solvent, applying the resist solution onto a substrate by a variety of methods, and evaporating off the solvent optionally by heating. The resist film is then exposed to radiation, for example, deep UV through a mask of a predetermined pattern. This is optionally followed by post-exposure baking (PEB) for promoting acid-catalyzed reaction. The exposed resist film is developed with an aqueous alkaline developer for removing the exposed area of the resist film, obtaining a positive pattern profile. The substrate is then etched by any desired technique. Finally the remaining resist film is removed by dissolution in a remover solution or ashing, leaving the substrate having the desired pattern profile.
The chemical amplification type, positive working, resist compositions adapted for KrF excimer lasers generally use a phenolic resin, for example, polyhydroxystyrene in which some or all of the hydrogen atoms of phenolic hydroxyl groups are protected with acid labile protective groups. Iodonium salts, sulfonium salts, bissulfonyldiazomethane compounds, N-sulfonyloxydicarboxyimide compounds and O-arylsulfonyloxime compounds are typically used as the photoacid generator. If necessary, there are added additives, for example, a dissolution inhibiting or promoting compound in the form of a carboxylic acid and/or phenol derivative having a molecular weight of up to 3,000 in which some or all of the hydrogen atoms of carboxylic acid and/or phenolic hydroxyl groups are protected with acid labile groups, a carboxylic acid compound for improving dissolution characteristics, a basic compound for improving contrast, and a surfactant for improving coating characteristics.
O-arylsulfonyloxime compounds as shown below are advantageously used as the photoacid generator in chemically amplified resist compositions, especially chemically amplified, positive working, resist compositions adapted for KrF excimer lasers because they provide a high sensitivity and resolution and eliminate poor compatibility with resins and poor solubility in resist solvents as found with the sulfonium and iodonium salt photoacid generators. See U.S. Pat. Nos. 6,004,724, 6,261,738, JP-A 9-095479, JP-A 9-208554, JP-A 9-230588, Japanese Patent No. 2,906,999, JP-A 9-301948, JP-A 2000-314956, and JP-A 2001-233842. 
As the requisite pattern size is reduced, however, even the use of these photoacid generators encounters problems including poor resolution and low stability to the environment.
With respect to the resolution, improvements are being made by rendering the acid labile groups in the resin more scissile by acid, using basic additives, or optimizing process conditions.
The environmental stability is generally divided into two categories. One stability problem is that the acid generated upon exposure is deactivated with airborne bases on the resist film or bases on the substrate beneath the resist film. This phenomenon is often found when a photoacid generator capable of generating an acid having a high acid strength is used. There is a probability that this problem will be solved by rendering the acid labile groups in the resin more scissile to acid or by reducing (or weakening) the acid strength of the acid generated. The other problem of environmental stability is that when the duration between exposure and post-exposure bake (PEB) is prolonged, that is, in the case of post exposure delay (PED), the acid generated diffuses through the resist film so that acid deactivation occurs if acid labile groups are less scissile or acid decomposition reaction occurs if acid labile groups are more scissile, often leading to variations of the pattern profile. In chemically amplified positive resist compositions having acid labile groups including primarily acetal groups, for example, the line width of unexposed areas is often narrowed.
As discussed above, for higher resolution, it is necessary to introduce more scissile acid labile groups into a resin, and it is desirable for the photoacid generator to generate a low diffusible acid. As the low diffusible acid, alkylsulfonic acids have been in study. Typical alkylsulfonic acids include 10-camphorsulfonic acid, butanesulfonic acid and octanesulfonic acid. Since all these alkylsulfonic acids have a weaker acid strength than fluorinated alkylsulfonic acids and arylsulfonic acids commonly employed in the prior art, the amount of acid must compensate for the weakness of acid strength. That is, a more amount of acid must be generated, which in turn, requires to extend the exposure time, often leading to a low productivity.
Under the circumstances, JP-A 2001-122850 describes sulfonium salts and iodonium salts capable of generating tosyloxybenzenesulfonic acid as the acid upon light exposure; and JP-A 2001-55373 describes tosyloxybenzenesulfonyl-diazomethane as a typical example. However, the former has the drawback (pattern sidewall roughening) associated with the use of onium salts and the latter has the drawback of difficult synthesis.
The photoacid generator for resist compositions is required to have a fully high solubility or compatibility in resist solvents and resins, good storage stability, non-toxicity, effective coating, well-defined pattern profile, PED stability, a high resolution, a wide focal depth, and a high sensitivity. Prior art photoacid generators, especially O-arylsulfonyloxime base photoacid generators do not satisfy all these requirements.
In the recent stage when the pattern of integrated circuits becomes more miniaturized, more stringent requirements are imposed on the problems of resolution and focal depth.